/*
 * Copyright 2002-2018 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.http;

import org.springframework.lang.Nullable;
import org.springframework.util.StringUtils;

import java.util.concurrent.TimeUnit;

/**
 * A builder for creating "Cache-Control" HTTP response headers.
 *
 * <p>Adding Cache-Control directives to HTTP responses can significantly improve the client
 * experience when interacting with a web application. This builder creates opinionated
 * "Cache-Control" headers with response directives only, with several use cases in mind.
 *
 * <ul>
 * <li>Caching HTTP responses with {@code CacheControl cc = CacheControl.maxAge(1, TimeUnit.HOURS)}
 * will result in {@code Cache-Control: "max-age=3600"}</li>
 * <li>Preventing cache with {@code CacheControl cc = CacheControl.noStore()}
 * will result in {@code Cache-Control: "no-store"}</li>
 * <li>Advanced cases like {@code CacheControl cc = CacheControl.maxAge(1, TimeUnit.HOURS).noTransform().cachePublic()}
 * will result in {@code Cache-Control: "max-age=3600, no-transform, public"}</li>
 * </ul>
 *
 * <p>Note that to be efficient, Cache-Control headers should be written along HTTP validators
 * such as "Last-Modified" or "ETag" headers.
 *
 * @author Brian Clozel
 * @author Juergen Hoeller
 * @see <a href="https://tools.ietf.org/html/rfc7234#section-5.2.2">rfc7234 section 5.2.2</a>
 * @see <a href="https://developers.google.com/web/fundamentals/performance/optimizing-content-efficiency/http-caching">
 * HTTP caching - Google developers reference</a>
 * @see <a href="https://www.mnot.net/cache_docs/">Mark Nottingham's cache documentation</a>
 * @since 4.2
 */
public class CacheControl {

    private long maxAge = -1;

    private boolean noCache = false;

    private boolean noStore = false;

    private boolean mustRevalidate = false;

    private boolean noTransform = false;

    private boolean cachePublic = false;

    private boolean cachePrivate = false;

    private boolean proxyRevalidate = false;

    private long staleWhileRevalidate = -1;

    private long staleIfError = -1;

    private long sMaxAge = -1;


    /**
     * Create an empty CacheControl instance.
     *
     * @see #empty()
     */
    protected CacheControl() {
    }


    /**
     * Return an empty directive.
     * <p>This is well suited for using other optional directives without "max-age",
     * "no-cache" or "no-store".
     *
     * @return {@code this}, to facilitate method chaining
     */
    public static CacheControl empty() {
        return new CacheControl();
    }

    /**
     * Add a "max-age=" directive.
     * <p>This directive is well suited for publicly caching resources, knowing that
     * they won't change within the configured amount of time. Additional directives
     * can be also used, in case resources shouldn't be cached ({@link #cachePrivate()})
     * or transformed ({@link #noTransform()}) by shared caches.
     * <p>In order to prevent caches to reuse the cached response even when it has
     * become stale (i.e. the "max-age" delay is passed), the "must-revalidate"
     * directive should be set ({@link #mustRevalidate()}
     *
     * @param maxAge the maximum time the response should be cached
     * @param unit   the time unit of the {@code maxAge} argument
     * @return {@code this}, to facilitate method chaining
     * @see <a href="https://tools.ietf.org/html/rfc7234#section-5.2.2.8">rfc7234 section 5.2.2.8</a>
     */
    public static CacheControl maxAge(long maxAge, TimeUnit unit) {
        CacheControl cc = new CacheControl();
        cc.maxAge = unit.toSeconds(maxAge);
        return cc;
    }

    /**
     * Add a "no-cache" directive.
     * <p>This directive is well suited for telling caches that the response
     * can be reused only if the client revalidates it with the server.
     * This directive won't disable cache altogether and may result with clients
     * sending conditional requests (with "ETag", "If-Modified-Since" headers)
     * and the server responding with "304 - Not Modified" status.
     * <p>In order to disable caching and minimize requests/responses exchanges,
     * the {@link #noStore()} directive should be used instead of {@code #noCache()}.
     *
     * @return {@code this}, to facilitate method chaining
     * @see <a href="https://tools.ietf.org/html/rfc7234#section-5.2.2.2">rfc7234 section 5.2.2.2</a>
     */
    public static CacheControl noCache() {
        CacheControl cc = new CacheControl();
        cc.noCache = true;
        return cc;
    }

    /**
     * Add a "no-store" directive.
     * <p>This directive is well suited for preventing caches (browsers and proxies)
     * to cache the content of responses.
     *
     * @return {@code this}, to facilitate method chaining
     * @see <a href="https://tools.ietf.org/html/rfc7234#section-5.2.2.3">rfc7234 section 5.2.2.3</a>
     */
    public static CacheControl noStore() {
        CacheControl cc = new CacheControl();
        cc.noStore = true;
        return cc;
    }


    /**
     * Add a "must-revalidate" directive.
     * <p>This directive indicates that once it has become stale, a cache MUST NOT
     * use the response to satisfy subsequent requests without successful validation
     * on the origin server.
     *
     * @return {@code this}, to facilitate method chaining
     * @see <a href="https://tools.ietf.org/html/rfc7234#section-5.2.2.1">rfc7234 section 5.2.2.1</a>
     */
    public CacheControl mustRevalidate() {
        this.mustRevalidate = true;
        return this;
    }

    /**
     * Add a "no-transform" directive.
     * <p>This directive indicates that intermediaries (caches and others) should
     * not transform the response content. This can be useful to force caches and
     * CDNs not to automatically gzip or optimize the response content.
     *
     * @return {@code this}, to facilitate method chaining
     * @see <a href="https://tools.ietf.org/html/rfc7234#section-5.2.2.4">rfc7234 section 5.2.2.4</a>
     */
    public CacheControl noTransform() {
        this.noTransform = true;
        return this;
    }

    /**
     * Add a "public" directive.
     * <p>This directive indicates that any cache MAY store the response,
     * even if the response would normally be non-cacheable or cacheable
     * only within a private cache.
     *
     * @return {@code this}, to facilitate method chaining
     * @see <a href="https://tools.ietf.org/html/rfc7234#section-5.2.2.5">rfc7234 section 5.2.2.5</a>
     */
    public CacheControl cachePublic() {
        this.cachePublic = true;
        return this;
    }

    /**
     * Add a "private" directive.
     * <p>This directive indicates that the response message is intended
     * for a single user and MUST NOT be stored by a shared cache.
     *
     * @return {@code this}, to facilitate method chaining
     * @see <a href="https://tools.ietf.org/html/rfc7234#section-5.2.2.6">rfc7234 section 5.2.2.6</a>
     */
    public CacheControl cachePrivate() {
        this.cachePrivate = true;
        return this;
    }

    /**
     * Add a "proxy-revalidate" directive.
     * <p>This directive has the same meaning as the "must-revalidate" directive,
     * except that it does not apply to private caches (i.e. browsers, HTTP clients).
     *
     * @return {@code this}, to facilitate method chaining
     * @see <a href="https://tools.ietf.org/html/rfc7234#section-5.2.2.7">rfc7234 section 5.2.2.7</a>
     */
    public CacheControl proxyRevalidate() {
        this.proxyRevalidate = true;
        return this;
    }

    /**
     * Add an "s-maxage" directive.
     * <p>This directive indicates that, in shared caches, the maximum age specified
     * by this directive overrides the maximum age specified by other directives.
     *
     * @param sMaxAge the maximum time the response should be cached
     * @param unit    the time unit of the {@code sMaxAge} argument
     * @return {@code this}, to facilitate method chaining
     * @see <a href="https://tools.ietf.org/html/rfc7234#section-5.2.2.9">rfc7234 section 5.2.2.9</a>
     */
    public CacheControl sMaxAge(long sMaxAge, TimeUnit unit) {
        this.sMaxAge = unit.toSeconds(sMaxAge);
        return this;
    }

    /**
     * Add a "stale-while-revalidate" directive.
     * <p>This directive indicates that caches MAY serve the response in which it
     * appears after it becomes stale, up to the indicated number of seconds.
     * If a cached response is served stale due to the presence of this extension,
     * the cache SHOULD attempt to revalidate it while still serving stale responses
     * (i.e. without blocking).
     *
     * @param staleWhileRevalidate the maximum time the response should be used while being revalidated
     * @param unit                 the time unit of the {@code staleWhileRevalidate} argument
     * @return {@code this}, to facilitate method chaining
     * @see <a href="https://tools.ietf.org/html/rfc5861#section-3">rfc5861 section 3</a>
     */
    public CacheControl staleWhileRevalidate(long staleWhileRevalidate, TimeUnit unit) {
        this.staleWhileRevalidate = unit.toSeconds(staleWhileRevalidate);
        return this;
    }

    /**
     * Add a "stale-if-error" directive.
     * <p>This directive indicates that when an error is encountered, a cached stale response
     * MAY be used to satisfy the request, regardless of other freshness information.
     *
     * @param staleIfError the maximum time the response should be used when errors are encountered
     * @param unit         the time unit of the {@code staleIfError} argument
     * @return {@code this}, to facilitate method chaining
     * @see <a href="https://tools.ietf.org/html/rfc5861#section-4">rfc5861 section 4</a>
     */
    public CacheControl staleIfError(long staleIfError, TimeUnit unit) {
        this.staleIfError = unit.toSeconds(staleIfError);
        return this;
    }


    /**
     * Return the "Cache-Control" header value, if any.
     *
     * @return the header value, or {@code null} if no directive was added
     */
    @Nullable
    public String getHeaderValue() {
        String headerValue = toHeaderValue();
        return (StringUtils.hasText(headerValue) ? headerValue : null);
    }

    /**
     * Return the "Cache-Control" header value.
     *
     * @return the header value (potentially empty)
     */
    private String toHeaderValue() {
        StringBuilder headerValue = new StringBuilder();
        if (this.maxAge != -1) {
            appendDirective(headerValue, "max-age=" + this.maxAge);
        }
        if (this.noCache) {
            appendDirective(headerValue, "no-cache");
        }
        if (this.noStore) {
            appendDirective(headerValue, "no-store");
        }
        if (this.mustRevalidate) {
            appendDirective(headerValue, "must-revalidate");
        }
        if (this.noTransform) {
            appendDirective(headerValue, "no-transform");
        }
        if (this.cachePublic) {
            appendDirective(headerValue, "public");
        }
        if (this.cachePrivate) {
            appendDirective(headerValue, "private");
        }
        if (this.proxyRevalidate) {
            appendDirective(headerValue, "proxy-revalidate");
        }
        if (this.sMaxAge != -1) {
            appendDirective(headerValue, "s-maxage=" + this.sMaxAge);
        }
        if (this.staleIfError != -1) {
            appendDirective(headerValue, "stale-if-error=" + this.staleIfError);
        }
        if (this.staleWhileRevalidate != -1) {
            appendDirective(headerValue, "stale-while-revalidate=" + this.staleWhileRevalidate);
        }
        return headerValue.toString();
    }

    private void appendDirective(StringBuilder builder, String value) {
        if (builder.length() > 0) {
            builder.append(", ");
        }
        builder.append(value);
    }


    @Override
    public String toString() {
        return "CacheControl [" + toHeaderValue() + "]";
    }

}
